1. Technical Field
The present invention relates to earth boring bits, and more particularly to those having polycrystalline diamond compact (PDC) cutters.
2. Description of Related Art
Efficiently drilling rock of various hardness or in overbalanced formations has always been related to the amount of power (RPM×WOB) injected in the drilling system (RPM=revolutions per minute; WOB=weight on bit). A linear relationship between ROP (rate of penetration) and WOB has always been taken into consideration for PDC bit performance, and cutting structure efficiency ranking can be evaluated through an examination of MSE (mechanical specific energy). Generally, this brought about the usage of high forces in order to be efficient. Usage of high cutting forces, however, can cause problems like BHA (bottom hole assembly) buckling, deviation issues, and dynamic problems resulting at the end in an inefficient usage of the power input to the drilling system. In addition, the usage of these high forces can induce on the cutting element itself premature failures due to potential impacts of various magnitude or frequency and higher frictional heat resulting in a faster cutting element wear rate.
PDC cutters are typically formed from a mix of material subjected to high temperature and high pressure. A common trait of a PDC cutter is the use of a catalyst material during their formation. These cutters are known to have several different shapes and geometries.
A PDC cutter with improved durability uses an elliptical shape. These cutters have been marketed as “oval” cutters. These cutters have an elliptical form (with a major axis and a minor axis). An elliptical cutter has a better indentation action than a round cutter. Thus, these elliptical cutters generate a more concentrated crushed zone in the formation and deeper tensile cracks in the surrounding non-crushed zone.
A conventional PDC cutter is placed with the diamond table facing the direction of bit rotation. The edge of the cutter is pushed into the formation by the WOB. When an elliptical cutter is used, the small end of the cutter (in the direction of the major axis) is typically presented to the formation. This has the effect of presenting a “sharper” edge, which generates a higher point loading at a lower WOB versus a round cutter. By replacing a 13 mm round PDC cutter by a 19*13 mm elliptical PDC cutter, the diamond volume (density or radial diamond content) of the cutter remains the same, but the cutter exposure and axial diamond volume can be increased significantly.
There is a need in the art for a PDC cutter having a configuration of its cutting structure which increases drilling efficiency (presenting a lower MSE level). For example, there is a need for a specific cutter shape and configuration that requires less WOB than conventional cutters for a given ROP, thus lessening the wear rate (thermal and dynamic) and further resulting in a higher cutting efficiency which brings about a higher ROP and durability. This cutting structure could thus be considered to be “sharper” than that of the prior art. Additionally, there would be an advantage if this improved cutting structure presented better diamond table cooling and an easier evacuation of cutting chips during operation.
The following references are incorporated herein by reference: U.S. Pat. Nos. 4,538,690, 4,558,753, 4,593,777, 4,679,639, 4,784,023, 5,078,219, and 5,332,051; and U.S. Patent Application Publication Nos. 2005/0247492, 2005/0269139 and 2007/0235230.